Magnetic analysis-apparatus



2 y Ruurd 1'. zuscHLAG MAGNETIC ANALYSISVAPPARATUS Filed March 23', 1934 INVENToR Tb eco/of ,z/scb/ag ATTORNEYS incorporated both the f Referring to ng. 1,111; win be seemed that two pv1o :substantially identical primary coils. i and 2, of

'relatively low ohmic resistanceare connected in Series with a voltage supply transformer 3, an ammeter 4, and a low ohmic non-inductive fixed resistance 5.. The primary coils are preferably constructed of a suitable number of turns of insulated copper wire of low ohmic resistance. The voltage supply transformer is preferably of the adjustable auto-transformer type. The input lleads of the transformer are connected to a suitable alternating current source 6.

'A pair of substantially identical `nacondary coils i 3 and I4, preferably composed of a suitable number oi turns c-f insulated copper wire of low ohmic resistance, are connected in series oppcw sition to the contact arms of a double-pole threeposition switch i5. The pair oi secondary coils are disposed in inductive relationship with the primary coils. preferably by placing each secondary coil within a primary coil in axial alignment therewith. By connecting the two potes of switch l5 to contact points il and I8 the two secondary coils are connecti'ole directly to tte end points of a potentiometer l5. .Placing the double-pole switch on the contact points 2i and 22 serves to connect the seconder'f coils in series with the fixed resistor i9. the e:.d points oi' potentiometer i6 and the variable resistor 26. Ey means of the single-pole triple-throw switch 25, the slider- 23 of the potentiometer l5 is conneczible through contact point 24 to a point on the direct wire between the secondary coils, this point being hereinafter referred to as the common point 48. The slider 23 oi potentiometer l may also be connected to the common point 48 through fixed resistance 26 and contact point 27 cf switch 25.

One end of the xed resistor 5 of the primary circuit is connected ln series with the milliammeter l to an end of a center tap potentiometer 8. The other end of xed resistor 5 is connectible to the other cxtrernity'of potentiometer 8 through a single-pole double-throw switch i2, either directly through contact point 9, or in series with a iixed resistance i0 through contact point Il.

The slider 49 oi center tap potentiometer 8 is connected to an end oi' potentiometer i6, thisv el i of potentiometer I6 being also connectible through contact points I8 or 22 to the secondary coils as has been described above. The other end oi' potentiometer I6, in addition to being connectlble with the secondary coils throughcontact points i 1 or 2i, is connectible in series with an inductance 28 and a capacitance 25 to the slider 3| oi a potentiometer 32. The inductance 28 and capacitance 2S serve the function of a filter and are preferably constructed to pass only the first harmonic of the primary circuit, butmay, if desired,'be constructed to pass the third 'or any other harmonic.

' The center tap of potentiometer 8 is connected `inparallel with a suitable current indicating Lmeans 41 and an end of potentiometer 32, the other end oi potentiometer 32 being connected in series to the second lead of the indicating means.

The functions o! the various parts o! the apparatus may be understood more easily by couside'ring the various modes of operation in connection with the drawing. 4In all cases the secondary coils i 3 and i4 are iirst tested for balance with no magnetizable bodies inserted in the fields o1 the primary and secondary coils. Toy accomplish this, switches I2 and 25 are placed in the center or oit position and switch i Sis closed over contact points il and i8. With the apparatus connectedvin this manner the potentiometer Ii is connected across the secondary coil circuit but slider 23 is not connected to the common point 48 of the circuit, so that any voltage drop across the potentiometer must be due to unequal electromotive forces induced in the'secondary coils. A voltage drop acrosslpotentiometer i6 causes current to iiow in the circuit i6, 28, 29, 3i, 32, 8 and, which is a closed circuit depending for energization upon the current flows in the several potentiometers. The existence of any current ilow inthis circuit is immediately indicated by a deection o! the indicator 4l, which shows the voltage imposed between slider .3| and the cornmon end of the potentiometer 32. It should be observed that in the case of properly designed secondary coils which are located coaxialiy with respect to the primary coils, the resultant deection of the indicator should be small.v

In order to compensate for any deection oi the indicator, switch ls closed on contact point 24, thus connecting coils i3 and i4 in bridge relation ship with the potentiometer. This bridge arrangement permits a variation of the ratio oi' the secondary currents induced in coils i3 and H. Thus the strength of the secondary potentials in these coils may be varied by adjusting slider 3l of potentiometer 32, while an adjustment of slider 23 of potentiometer I6 permits a reduction in the strengths of the potentials induced in the secondary coils to minimum values. In many instances, however, the minimum value attainable by the adjustment of potentiometer i6 is not zero, and in order to obtain a zero reading of the indicator it is necessary toclose switch I2 to either oi.' the two contact points ii and 9 and adjust the slider 49 oi potentiometer 8. 'It will be observed that the auxiliary drop thus imposed across potentiometer 8 between the center tap and the slider i9, is substantially 90 out oi phase with the open circuit air core potentials of the secondary cores. This is due to the source oi' this potential drop in the primary circuit. Secondary phase components in the secondary coil circuit which areso disposed as to be unaffected by adjustment of potentiometer i6, may bc neutralized by means oi' the 90" phase displaced drop of the potentiometer 3.

The range of compensation obtainable by potentiometers 8 and I6 may be varied by inserting resistances ID andY 26 into their respective circuits by mcans oi switches l2 and 25, and this increased range of compensation is obtained without altering the sensitivity of the apparatus to an appreciable degree. It should 'ne understood that while the resistors Hl and 26 are not absolutely essential to thc operation oi' the compensating netfwork. they are desirable ndiuncts because o! tlie additional range ot compensation which they vprovidel Usually in the case oi' sir core testing such) as hasbcen described, e well designed apparatus will require little compensation and zero readings will be obtain'cdwith the sliders close` to the mldpoints of potentiometer; il and I.

When thc indicator reads zero with no malnetizable bodies in the field oi primary and sec-r paring samples yfrom the same heat, the following procedure is followed;

The sliders of the potentiometers 8 and i8 are left unchangedA from the position which they occupied when Ythe indicator showed'zero reading*v for air core conditions. A standard A of known properties is inserted in the ,fieldiof one pair of primary and secondary coils, say-ifi .and I3, and

a lspecimen B, Iwhose properties are to` be determined, is inserted in the eld of thel other pair Aof primary and secondary coils, say Zand I4.

The indicator will be deflected in proportion to the differences in the magnetic properties of the standard and the specimen. The reading of the indicator is noted. If several specimens are to be tested these are substituted successively for specimen B in the coils, and the deflection correspending to each specimen is noted. These readings are indicative of the differences in metallurgical and/or physical properties between specimen and standard, or between specimens themselves.

When specimens and standard differ so marl:- edly in magnetizable characteristics as to cause a deection which is too great to be 'read on the 'scale of the indicator, as is frequently the case when specimens and standard represent different heats of metal, or when either specimens or standard contain flaws of considerable magnitude, the testing procedure is somewhat different. The change in procedure is made necessary by the fact that the setting of potentiometer iS fora zero reading under air core conditions must be changed in order to accommodate the reading upon the scale of the indicator. The procedure in such a case is as follows:

A standard A and a specimen B are inserted in the field of the coils as previously described,` and the resultant deilection of the indicator is reduced to zero by adjustment' of the potentiometers 8 and i6. The settings of the potentiometers 8 and i8 are noted. Specimen B is replaced by a. second specimen C and the potentiometers 8 and i6 are again adjusted until a zero reading is obtained. The settings of the potentiometers for specimen C are noted. vA comparison of the settings for the two cases serves as a basis for differentiating specimens B and C, insofar as their magnetic characteristics are concerned. It will be observed that this method of procedure, while producing results which are as trustworthy as those obtained in the previous method, is considerably slower because it requires considerable manipulation. For this reason it is preferable to choose a standard `whose magnetizable characteristics approximate those of the specimens to be tested, especially in the case of `quantity production. i

The procedures thus far described rind their chief utility in the qualitative testing of steel specimens where one or more defects of a particular kind are sought to be discovered by comparison with a standard in which these defects do 'not occur. The utility of the apparatus is not limited, however, to qualitative" determinations. The apparatus may also be used to establish a quantitative comparison of magnetizable bodies in that the total magnetic locs difference and the ratio oi the apparent pcrmeabilitics of two samples may be quantitatively determined with a high degree of accuracy. The procedure, in

.the lcenter tap and the slider.

the event that these'quantities are sought, is

as follows:

The two samples to be compared are inserted.

respectivelyinto the pairs of secondary and primary coils. The circuit is then balanced by adjusting potentiometer-s 8 and i6 to obtain com# plete neutralization, when the indicator will register zero.4 When this condition is obtained itis necessary to determine the value of the resistanceyot the section'of potentiometer 8 between Ordinarily, in' the case of a graduated potentiometer this is a matter of simple reading. It is also necessary to determine the root-mean-square value of the current owing through the potentiometer 8. Having determined these values for a balanced condition, the determination of total loss differences is simply a matter/of substituting the values in the following equation and solving it for W; i. e.

where; W is the total magnetic loss difference between the two samples, expressed in watts A iis the effective current flowing in potentiometer 8 in amperes and, r is the resistance between the center point of the potentiometer 8 and the slider 69, in ohms.

The determination of the permeability ratio is somewhat more complicated than the determination of the total loss diierence between the samples. It is based upon the determination of ratio of the secondary potentials induced in coils i3 and i4. In order to determine this potential natio, two successive compensations, for slightly different circuit conditions, are necessary. These two 'compensations are accompushed in the .'olowing manner:

The samples to be compared are inserted in the elds of the secondary coils.' The contact arms of the switch l5 are connected to contact points il and i8 and the contact arm oi' switch 25 is connected to the contact point 24. With In equals the resultant current in secondary coil IS;

b is equal to the resistance between contact point l1 and slider 23 of potentiometer IG; A

m equals the resistance of secondary coil il: the other resistances being indicated by small letter r and index number corresponding to the number of the apparatus on Fig. 1. The Iollowing equations then obtain:

Substituting thcvalues of In and In from Equa tions 1 and 2 in Equation 3:

It willbe'noted that Equation 4 contains the unknown impedances rn and m which must be eliminated before the numerical value ol the ratio E14/En can bc calculated. In order to so# complish this elimination; it is necessary to obl l tain a second series of values for another oompensated condition. This second compensation values are designatedas in These equations may be combined as follows:

` Equations 4 and 8 may now be combined to obtain'the ratio of the induced potentials in coils I3 and I4 in the following manner.:

i Fari; L' Eu fie-*b fro It wi'l be observed that Equation 9 contains no unknown factors since the values b, ria. ne and rca are known. The numerical value of the equation may therefore be easily computed. Knowing this value it is possible to compute, by means well known in the art, the ratio of the apparent permeability of the tested specimens.v

In conducting 'tests with the apparatus of my invention it is preferable that an eicient dlter combination 28, 29 be used. It is not essential that a filter of the particular type shown be employed, but whatever the type of rllter, the aim is to eliminate all but one harmonic voi the base 'frequency in order to suppress harmonics whose presence do not aid in the determination oi magnetio characteristics. 1t has been found that most defects and properties which are sought t be determined by magnetic analysis appear in the first. and third harmonics of the base irequency. Accordingly filters adapted to pass only one'oi these two frequencies are preferable. It should be noted, however, that the apparatus is notlimited to an investigation oi the basic and third harmonics of the exciter current. Occasionally it may be desirable to investigate other harmonics, and this may bev easily accomplished by changing the inductance and the capacitance oi the filter 2B. 28.

An outstanding advantage accrues to the use oi the compensating means of this invention when it is employed yin combination with the two-cozn poncntvindicator oi the invention which is described in my co-pending United States application, Serial No. '116.941, filed March 23, 1934.

circuit incorporating both the compensating- Fig. 2 shows a compensating magnetic analysis means described and the two-component indicator of my aforementioned application. It will be observed that this two-component' indicator has been substituted for the indicating means 61 shown on Fig. l. A synchronous motor 3B derives powa;- irom the current source 6 ahead of the auto'transiormer 3, and is operably associated with commutator 31. The ends of the potentiometer 32.V previously referred to in the description o! Fig. Llare connected to brushes 33 and 34. the brushes being in, contact with contact rings 3l and 36, respectively, ot the synchronous comaccesso' l along contact segments ZS and l0.

mutator 31. When the synchronous motor is of' the two-pole type the ccmmutator 31 is equipped with two separate contact segments 39 and 4 0 which are connected to the contact rings and 38, respectively. Two pairs oi' diagrammatically opposed brushes 4I, 42, and #3, 44 are symmetrically disposed upon the contacts 39 and 4I and are connected to two DArsonval type galvanometers 45 and .46. 90 phase displacedand may be moved 'in unison When this two-component type ofllndicator is used the potential dropbetween the two ends of potential 82 may be divided into two components which are 90 phase displaced with respect to each other. The voltage drop between slider II and the one end of the potentiometer 32 is transierred to contact rings 35, 3E and return to the contact segments 3.9, 40. The brush pairs 4I, 42 and 43, 4A receive the rectified voltage from the contact segments and deliver it to the indicators 45 and 46. the one complete cycle is broken up into two currents 90 phase displaced, integrated and indicated with regard to their arbitrary origins. The simultaneous indication of the two phase displaced components permits the reconstruction of the voltage wave, a feature which normally may be accomplished only with the help o! an oscillograph.

It will be apparent that when carrying out tests with the apparatus, including the two-cornponent indicator, it is essential to use an eiicient filter combination 28. 2S, in order to pass only the harmonic which is being investigated. In order to further describe the process involved in my new apparatus and method for magnetic testing in which a two-component inmcator is used in conjunction with the compensating means'which has been described. the following' is set forth:

Currents induced in secondary coils I3, I4 arev compensated first by connecting the coils in series opposition, and second, by adjusting the slider of potentiometer i 6. The resultant differential current, which is not necessarily in phase with the primary current, may be completely neutralized by an auxiliary potential obtained by adjusting the slide; of potentiometer 8. The state oi complete neutralization is indicated by zero readings on the two indicators which are connected in series with the segments of commutator 31. The commutator serves to break up the cycle diii'erential current' into two 90 phase displaced currents4 which are separately integrated and shown by the indicators. True neutralization is obtained only when both the indicators read zero. By means of the two-component indicator it is possible to reconstruct the wave form o1' the dit i'erential current in a manner which previously was impossible without the use o1 oscillographs.

When the two-component indicator is used in conjunction with the compensating net-work of In other words, the voltage wave o!l The two b'rush pairs are my invention it is possible to determine by means of one indicator the presence of a total loss difterence and its magnitude, while the companion indicator will reflect the ratio of the potential differences inducedin the two secondary coils.

Investigation has shown that the readings of the two. indicators may be weil calibrated by comparing them with the potentiometer readings, so that accurate values of both potential ratio and total loss difference may be determined directly on the indicators. .f

I claim:

i. Apparatus for magnetic analysis which com- `a pair of secondary coils are placed in inductive relationship with a source of alternating current and connected in series opposition in a secondary circuit with an indicating means, the improvement which comprises a bridge arrangement in said secondary circuitcontaining adjustable resistances and means for connecting said secondary circuit with the primary current source soA that primary current may be introduced into said secondary circuit to neutralize secondary differential currents induced therein. y

3. In apparatus for magnetic analysis the improvement which comprises a secondary circuit containing coils lnductlvely associated with an alternating current source and with magnetizable bodies to be tested and connected in series opposition by means of a bridge network containing an adjustable potentiometer and a current indicating means, and means for connecting the alternating current source and said po.- tentiometer whereby primary Acurrent may be introduced to neutralize secondary currents induced by extraneous forces and variations between the magnetizable bodies which tend to mask the true diderences in magnetic characteristics between said magnetizable bodies.

4. Apparatus for magnetic analysis comprising a. pair of primary coils, means for connecting the primary coils in series with an alternating current source through an adjustable transl former, a pair oi secondary coils disposed respectively in inductive relationship with said primary coils, a bridge circuit by which said secondary coils are connected in series opposition with a current indicatingmeans, said bridge circuit containing one or more adjustable resistences, and means for connecting said bridge circuit with the alternating current souroe'so that auxiliary currents may be introduced into the bridge circuit.

5. Apparatus for magnetic analysis comprising a pair of substantially identical primary coils connected in series with a fixed resistance and an alternating current source, a pair oi secondary coils disposed in inductive relationship respectively with said primary coils and connected in,

. series opposition with a current indicating means through a bridge circuit, said secondary coils being adapted to be located in inductive relationship with magnetiaable bodies to be tested, and means for connecting said bridge circuit with the primary circuit on either side of the fixed resistance so that current which is substantially in phase with the primary current may be introduced into the bridge circuit to compensate for 4the effect of differences between the magnetizable bodies and of extraneous i'orces which tend to interfere in the determination of true difierences in the physical characteristics of the bodies.

6.4 Apparatus formagnetic analysis which.. comprises a primary circuit in which two primary coils are connected in series with a source of al" ternating current and a fixed resistance, a pair of secondary coils inductively associated with saidl tiometer, the slider of which is connected to a common point on the connection between said secondary coils, a ,second potentiometer, means for connecting the ends of the second potentiometer with the primary circuit on both sides of the fixed resistance, means for connecting the` center pointI of the second potentiometer in parallel with a current indicating means and one end -of a third potentiometer the other end of which is connected with the current indicating means, a lead by means of which the slider oi the second potentiometer is connected to an end of the iirst potentiometer, and a second lead connecting the other end of the first potentiometer in series with the slider of the third potentiometer.

7. Apparatus according to claim 6 in which a. filter is connected in series with one end o1' the first potentiometer and the sliderA of the third potentiometer.

8. Apparatus according to claim 6 in which a fixed resistance is connected in series with the slider of the rst potentiometer and the common point between'the two secondary coils.

9. Apparatus according to claim 6 in which a fixed resistance is connected in series between the end of one secondary coil and an end of the rst potentiometer, and a. variable resistance is connected in series between the other secondary coil and the other end of the rst potentiometer.

10. Apparatus according to claim 6 in which a xed resistance is'ccnnected in series between one end of the second potentiometer and an end of the xed resistance in the primary circuit.

11. Magnetic analysis apparatus which comprises a primary circuit for alternating current, a secondary circuit disposed in inductive relationship with said primary circuit and containing a compensating network connected with a two-ccmponent integrating current indicating means,

and operatively associated with a synchronous commutator to which are connected integrating galvanometers, two secondary coils disposed in inductive relationship with the primary coils and connected in series opposition with a bridge network and the synchronous commutator.

13. Magnetic analysis apparatus which comprises a primary circuit having two primary coils connected in series, means for connecting the primary coils to van alternating current source, a pair of secondary coils disposed in inductive relationship with the primary coils and connectedV in series opposition with each other and with the end points of a first potentiometer, means for connecting the slider of the iirst potentiometer with a point on the connection between the secondary coils, a second potentiometer, a current indicating means, means for connecting the ends of the second potentiometer with the alternating current source, means for connecting the center point of the second potentiometer with the current indicating means, a third potentiometer one end of which is connected to the midpoint of the second potentiometer, the other end o! the third potentiometer being connected with the current indicating means, means tor 'connecting the slider ot the second potentiometer with one end of the rlrstfpotentiometer. and means for connecting the other end of the ilrst potentimeter with the slider oi the third potentiometer.

14. Apparatus for magnetic analysis which comprises a pair of primary coils, means for con nectingsaid primary coils in series with an alter nating currentsource, a pairof secondary coils disposed respectively ininductive relationslrlp Vwith said primary coils and connected in series 2,056,9eo l s opposition in a bridge circuit, means for con-` necting the alternating current source to the bridge circuit, a synchronous commutator connected in :said bridge circuit. and a current indicating means connected with said synchronous commutator.

15. Apparatus according to claim 1 4 having s filter connected in `the bridge circuit, said filter being designed tosuppress lall but one hannonic of a secondary alternating current.

Tacoma, zUscHLAct.r 

